1. Technical Field
The present invention generally relates to raising and lowering roller shades, and more particularly to raising and lowering a roller shade to a selected position at a substantially constant linear velocity, and raising and lowering a plurality of roller shades synchronously.
2. Background Art
A typical motorized roller shade includes a flexible shade fabric wound onto an elongated roller tube. The roller tube is rotatably supported so that a lower end of the flexible shade fabric can be raised (i.e., wound) or lowered (i.e., unwound) by rotating the roller tube. The roller tube is rotated by a motorized drive system.
A common problem with typical motorized roller shades is that when the shade is raised or lowered, the motorized drive system, which moves the shade at a constant velocity, abruptly starts rotating the shade, winds or unwinds the shade at the constant velocity, and then abruptly stops rotating the shade when the shade reaches a selected position. Consequently, during raising or lowering of the shade, the shade moves with an aesthetically unpleasing “jerky” motion. Further, sometimes the shade undershoots the selected position because the shade is abruptly stopped too early. Other times, the shade overshoots the selected position because the shade is abruptly stopped to late, or because the shade's momentum carries it past the selected position.
Attempts to position correctly a roller shade have included counting the rotations of the shade motor while the shade moves at a constant linear velocity. The linear velocity of a roller shade is typically estimated by determining the rotations per minute (RPMs) of the shade motor and multiplying the RPMs by the estimated changing distance between the last outer layer of fabric rolled on the shade tube and the tube center as the shade fabric is rolled or unrolled. This indirect method of determining linear velocity does not account for variations in shade fabric thickness and the random gaps that develop between the layers of the shade fabric. The accuracy of the positioning of the shade is limited by the accuracy of the motor rotational position measurement.
Another common problem with motorized roller shades is that when multiple roller shades are used to shade a room, and all the shades are raised or lowered at the same constant velocity, there is no guarantee that all the shades will arrive at a selected position at the same time, which is also aesthetically unpleasing.
For example, if one shade is longer than other shades in the same room (e.g., because the shade covers a longer window), the longer shade, moving at a constant velocity, will arrive at the selected position some time after the shorter shades have arrived at the selected position (e.g., all shades moving from the fully closed position to the fully open position). Likewise, if all the shades in a room are of equal length, but are each in different starting positions, each shade, moving at a constant velocity, will arrive at the selected position at a different time.
Therefore, a need exists for a motorized roller shade that starts and stops smoothly while not undershooting or overshooting the selected shade position. Additionally, a need also exists for a motorized roller shade that allows each of a plurality of shades to raise or lower at varying velocity so that each of the plurality of shades arrives at the desired position at the same time.